The invention of this application involves an imaging system which is based on an orbiting satellite, the quality of the image depends on the systems ability to point to the target and the stability of the satellite platform during the image generation process. The image consists of a swath across the target and must be assembled as a mosaic by the image processing software. The optical system onboard the satellite consists of a camera and associated mirrors through which the image is received. The imaging instrument is adjusted by altering the position of the mirrors and by controlling the attitude of the space craft. The mirrors are gimbal mounted to provide a two axis adjustment relative to the target which may be defined by longitude and latitude if the target is on earth. The pointing function requires position and attitude data which are, in prior art systems, provided through the use of precision star and earth sensors. A great deal of effort has been invested to maintain the positional stability of the satellite platform resulting in increased complexity in the associated attitude control system.
In general, space craft attitude is adjusted by activating actuators, such as, momentum wheels, magnetic torguers, or thrusters in response to an attitude correction signal. The attitude error is sensed by reference to sensors monitoring the position of the sun, stars and earth relative to the satellite. The attitude is adjusted to its mission orientation in which the imaging system is pointed at its predetermined target and is maintained in this orientation during orbital flight. During flight the satellite is subject to motions induced by external forces, on board mechanisms or other sources and the attitude control system must continuously monitor and adjust attitude.
In addition the line of sight of the imaging instrument must be maintained in registration with the target image. This is accomplished by movement of the mirror on its gimbal mounting. This adjustment is generally a finer adjustment and is less jarring to the space craft than actuator activation. The adjustment is accomplished by actuating servo motors in appropriate increments. A system of controlling the mirror position relative to the target is described in U.S. Pat. Nos. 4,688,091 and 4,688,092. The system described is an earth based system which periodically transmits attitude and orbit position data to the satellite from which the onboard control computer calculates positional errors and translates such errors into mirror position corrective adjustments. The position data is based on star and landmark data obtained from the imaging instrument and transmitted to earth. This data is updated periodically, every 24 hours, and used in attitude and orbit models to estimate attitude and orbital position from which the error calculations are made by the onboard computer.
Although the system of the above referenced patents provide an accurate control of image registration, it has been found that further errors occur because of non-repeatable perturbations which cannot be predicted by the models. The models are based on repetitive attitude disturbances, such as solar stresses, which occur as the position of the sun changes, the models do not respond to random disturbances such as cloud motion, thermal snapping and others. One solution is to update the model more frequently for example 5 or 6 times daily, but the frequency is limited by the availability of data from the image system. Since the mirrors must be slewed to obtain star data during intervals when the imaging process is dormant, there are inherent delays in obtaining the data.
The errors that accumulate have been calculated to be in the range of 50 arc seconds in a day which translates to approximately 10 kilometers on the surface of the earth. It is desirable to limit such errors to below 5 arc seconds in a day. It is the purpose of this invention to provide a supplemental error correction system which fine tunes the mirror position to compensate for errors caused by non-repeatable disturbances.